The present invention relates to a projection apparatus for a head-up display, to a head-up display having such a projection apparatus, and to a vehicle having such a head-up display.
A head-up display in a vehicle is tasked with visually representing information within the natural field of view of the driver. The natural field of view corresponds to the vehicle's surrounding area, which the driver must primarily observe during the operation of the vehicle, e.g. the road section in front of a car. The relevant information is here projected by a projection apparatus onto a suitable projection surface, e.g. a windshield, and reflected here such that the driver can visually perceive the information without having to move his gaze away from the surrounding area that is to be primarily observed during the operation of the vehicle, which significantly increases the operational safety of a vehicle that is equipped with such a head-up display.
In order to ensure reliable function in different situations, in particular under varying illumination conditions, it is necessary in a head-up display to take measures to keep light that is incident from outside the vehicle, in particular sunlight, out of the beam path of the projection apparatus or at least to reduce the penetration of light. Light that is reflected at the surfaces of optical elements of the projection apparatus otherwise results in reflections which can become noticeable within the field of view of the driver and, in an extreme case, can dazzle him, or at least irritate him and/or superimpose on the information to be visualized and possibly render said information unreadable.
It is the object of the present invention to specify a projection apparatus for a head-up display, a head-up display having such a projection apparatus, and a corresponding vehicle, in which the occurrence of disturbing light reflections is reliably avoided or at least reduced.
The projection apparatus according to the invention has an imaging unit, which is designed for producing an image, and a projection unit, which is designed for projecting the image, which has been produced by the imaging unit, via a projection path onto a projection surface, and is characterized by a curved optical element arranged in the projection path, said curved optical element being provided with a circular polarizer or being designed as a circular polarizer.
The head-up display according to the invention has a projection surface, in particular a front window of a vehicle, and a projection apparatus according to the invention.
The vehicle according to the invention, in particular motor vehicle, has a projection surface, in particular in the form of a front window, and a projection apparatus according to the invention.
The invention is based on the approach of providing a curved circular polarizer, or a curved optical element that is provided with a circular polarizer, in the projection path. The curved circular polarizer, or the curved optical element that is provided with the circular polarizer, is situated between the projection surface, in particular a windshield, and the imaging unit, for example a display. The curvature of the surface of the circular polarizer, or of the optical element that is provided with the circular polarizer, is preferably concave on the side that faces the projection surface. The curved circular polarizer, or the curved optical element that is provided with a circular polarizer, is shaped and/or arranged in the projection path in a manner such that some of the unpolarized light that is incident from the projection surface, such as sunlight, is reflected on the concave side in a manner such that it is not returned onto the projection surface. This prevents this component of the light, which is reflected at the circular polarizer, or at the optical element that is provided with the circular polarizer, from appearing within the field of view of the driver in the form of a disturbing reflection. However, since the curved circular polarizer, or the curved optical element that is provided with the circular polarizer, must be transmissive at least in a specific wavelength range for the image that is produced by the imaging apparatus and is projected onto the projection surface by the projection apparatus, it is unavoidable that a portion of the unpolarized light that is incident from the projection surface passes through the circular polarizer, or through the optical element that is provided with the circular polarizer. During passage through the circular polarizer, a first polarization direction is imparted on said component of the unpolarized light that is incident from the projection surface, wherein the mutually perpendicular components thereof are delayed by a phase of π/2, such that it is circularly polarized in the interior of the projection apparatus. After a reflection at a surface of an optical element which is situated in the projection apparatus, e.g. a mirror or a lens, the circular polarizer, or the optical element that is provided with the circular polarizer, now acts in the opposite propagation direction of the circularly polarized light as an absorber, because the phase delay of the two mutually perpendicular components now produces a second polarization direction, which is perpendicular to the first polarization direction. The circular polarizer, or the optical element that is provided with the circular polarizer, consequently prevents exit of the light, which is reflected at surfaces of optical elements of the projection apparatus, in the direction of the projection surface, with the result that any occurrence of reflections within the field of view of the driver is prevented or at least considerably reduced due to the light that is reflected at surfaces of the optical elements situated in the projection apparatus.
Overall, the invention consequently prevents the components of light, in particular of sunlight, which is incident on the projection apparatus from outside, i.e. both the component that is reflected at the curved circular polarizer, or at the curved optical element, and the component that is reflected at further optical elements in the interior of the projection apparatus, from appearing within the field of view of the driver in the form of disturbing reflections. The formation of undesired reflections in the natural field of view of the driver is hereby prevented or at least significantly reduced.
It is advantageous if, as an additional measure for the reduction of disturbing reflections, the surfaces of the optical elements situated in the projection apparatus are provided with anti-reflection coatings or anti-reflection surface structures. The component of the light that is reflected in the interior of the projection apparatus, in particular at the surfaces of optical elements in the projection path of the projection apparatus, and, upon another passage through the circular polarizer, must be absorbed thereby, is consequently reduced. The occurrence of disturbing reflections in the natural field of view of the driver is hereby even more reliably suppressed or reduced.
It is furthermore advantageous if the curved circular polarizer, or the curved optical element that is provided with the circular polarizer, is arranged such that light that is incident from the projection surface is steered, by reflection at the concave side of the circular polarizer, or of the optical element that is provided with the circular polarizer, onto a beam dump, e.g. an absorbing optical element. This reliably prevents light that is incident from the projection surface and is reflected by the concave surface of the circular polarizer, or of the optical element, from being incident on the projection surface and from being perceived there by the driver as a disturbing reflection.
Alternatively or additionally, the curved circular polarizer, or the curved optical element that is provided with the circular polarizer, is designed and/or arranged such that light that is coming from the region of the driver's eyes, is reflected at the projection surface and is incident on the curved element is steered, by reflection at the concave side of the circular polarizer, or of the optical element that is provided with a circular polarizer, onto a beam dump, e.g. an absorbing optical element. As a result, light that is coming from the region of the driver's eyes (aka eyebox) is also absorbed by the beam dump such that it is reliably ensured that all beam paths that would cause disturbing reflections in the region of the driver's eyes are suppressed.
In a preferred embodiment, the circular polarizer includes a linearly polarizing element and a quarter-wave layer. Unpolarized light that is incident on the linearly polarizing element in particular from outside the vehicle has, after passage through the linearly polarizing element, only a single oscillation direction, i.e. a fixed polarization. The quarter-wave layer is oriented such that two mutually perpendicular components of the light that is polarized in this oscillation direction are delayed upon passage through the quarter-wave layer such that their phase difference is exactly π/2, as a result of which the oscillation direction of the now circularly polarized light has, as viewed from a spatially fixed point, the form of a helix. It is thus ensured in a simple manner and with high reliability that incident unpolarized light is circularly polarized upon penetration into the projection apparatus.
In a further preferred embodiment, the quarter-wave layer is arranged, as viewed in the direction of the projection path during the projection of the image onto the projection surface, upstream of the linearly polarizing element. Light that is incident in the projection apparatus from outside consequently first passes through the linearly polarizing element and then the quarter-wave layer and is thereby circularly polarized in a simple and reliable fashion. Conversely, the light which is subsequently reflected in the interior of the projection apparatus at surfaces of optical elements passes, before it exits the projection apparatus, first the quarter-wave layer and then the linearly polarizing element. The circular polarizer, or the optical element that is provided with the circular polarizer, acts as an absorber due to the now opposite propagation direction of the circularly polarized light, because the phase delay of the two mutually perpendicular components of the light due to the quarter-wave layer now produces a second polarization direction, which is perpendicular to the first polarization direction, with the result that the light which is thus linearly polarized is absorbed by the linearly polarizing optical element that is situated therebehind. As a result, exit of the light, which has been reflected at surfaces of optical elements in the projection apparatus, in a direction of the projection surface is prevented or at least considerably reduced.
In a further preferred embodiment, the optical element is designed as a curved, transparent plate, in particular a glass or plastics plate. The circular polarizer is attached to this curved, transparent plate, in particular laminated on. Due to this design of the optical element, a reliable circular polarization of the incident unpolarized light with simultaneous simple production is ensured. Moreover, an optical element which is designed in this fashion can be installed in a highly space-saving manner, i.e. only a small installation space is required.
In an advantageous embodiment, a first side of the optical element is concavely curved, and a second side of the optical element is convexly curved. The extent of the curvature on the first and second sides of the optical element is preferably identical. Consequently, the optical element is thin, lightweight and space-saving and can be easily installed in the projection apparatus.
In a further advantageous embodiment, the circular polarizer is attached to the second side, i.e. the convexly curved side, of the optical element, in particular laminated on. Consequently, the reflection properties of the first side, i.e. the concave side, of the optical element with respect to the light that is incident from outside can be optimized independently of the implementation of the circular polarizer, which for its part can be optimized in particular with respect to the polarizing or polarization-rotating properties independently of the design of the reflective first side of the optical element. Undesired reflections within the field of view of the driver are hereby particularly reliably prevented or at least reduced.
In a further advantageous embodiment, the first side, i.e. the concave side, of the optical element faces toward the projection surface. Optionally, the second side, i.e. the convex side, of the optical element faces the projection unit. The optical element is hereby particularly expediently aligned in order to at least partially reflect light, which is incident from outside and/or light which is coming from the region of the driver's eyes, is reflected at the projection surface and is incident on the first side of the optical element, in a manner such that it can no longer reach the projection surface and cause here disturbing reflections within the field of view of the driver. In a preferred embodiment, the concavely curved surface of the optical element has a focusing effect, such that reflected light is steered in a controlled fashion in a direction in which the occurrence of reflections that are perceived by the driver as dazzling or at least disturbing is particularly reliably avoided. Furthermore, a portion of the light which is incident on the projection apparatus from outside, such as e.g. sunlight, is hereby efficiently reflected at the first side of the optical element, even before the remaining portion of the light passes through the circular polarizer. Consequently, the intensity of the light which passes into the projection apparatus is decreased, such that undesired reflections within the field of view of the driver are particularly reliably prevented or further reduced.
In a particularly preferred embodiment, the projection apparatus has a beam trap, which is arranged and/or designed such that it swallows, in particular absorbs, light rays which are incident on and reflected at the first side of the optical element. What is hereby prevented with particular high reliability is that light which is incident from the outside and reflected at the optical element is visually perceived by the driver as a disturbing reflection. It is advantageous if the beam trap is arranged in the direct neighborhood of the curved optical element, in particular next to the curved optical element. In this way, the beam trap and the curved optical element can be installed in a space-saving manner.
In a further preferred embodiment, the projection unit includes at least one further optical element, in particular a mirror, a mirror system, a lens, a lens system or a light guide, wherein at least one surface of the at least one further optical element has an anti-reflective layer. As a result, the component of the circularly polarized light that is reflected at the at least one surface of the at least one further optical element is reduced, typically by a factor of 4 to 5. Undesired reflections in the field of view of the driver due to reflections of the light that penetrates the projection apparatus from the outside and is reflected here at surfaces of optical elements are in this way particularly reliably prevented or further reduced.
In a further, particularly preferred embodiment, the projection apparatus has a housing, which encloses the projection apparatus and has an exit window through which the image that has been produced by the imaging unit is projected onto the projection surface. The exit window is here preferably formed by the optical element. The optical element preferably acts as a cover glass of the housing. In these embodiments, the curved circular polarizer, or the optical element that is provided with a circular polarizer, serves not only to reduce or prevent disturbing reflections in the field of view of the driver, but moreover offers effective and easy-to-realize protection of the components of the projection apparatus against ingress of dirt and dust, which could restrict the ability of the projection unit to function.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.